Add wrapper mapping per form basis.

Fixes #931.

The motivation for this feature comes from the bootstrap upgrade from 2 to 3.

To preserve backward compatibility with bootstrap 2, we allow the user to specify a custom wrapper per form basis, without changing the defaults.

e.g 

```ruby
simple_form_for(@foo, wrapper_mapping: { checkbox: :bootstrap_checkbox })
```

Related to https://github.com/rafaelfranca/simple_form-bootstrap/pull/28#issuecomment-28152079